Many mobile applications count on an auxiliary power distribution system that converts energy from a variable input DC bus to a regulated output three-phase four-wire power system. Such auxiliary power distribution system units are already built as DC/DC/AC stand-alone units and they equip certain vehicles.
Parallel operation of inverters to deliver more power across the load was the subject of intense research effort. Previous solutions propose paralleling at the insulated gate bipolar transistor level, at the inverter leg level, or after the line impedance. Since conventional units are packaged and located on different vehicles, their parallel connection should happen outside the main converter box, after the filter. For this situation, the industry's most used solution relies on the droop coefficient method. This method calculates the active and reactive components of power at the point of load and modifies both the magnitude based on reactive power components and phase (or frequency) based on active power component to provide power sharing. This modifying of the frequency based on the active power component constitutes a serious shortcoming when working with loads that require fixed, constant frequency for proper operation.
Different standards for power systems specify the electronic power converter to maintain frequency regulation within 0.25% of the setting as opposed to approximately 3% for mechanical governors. When using a single power converter to constitute a power source, the steady-state frequency can easily be maintained within the required range. When connecting multiple inverter-based power sources in parallel, this control strategy will result in oscillating power on the common bus each time there is a phase difference between the two power sources. Even the use of a closed-loop approach to frequency regulation and phase adjustment (e.g. PLL type) would lead to a conflict between the two or more controllers. The compromise solution consists in introducing a droop characteristic which means that the frequency is not restored to its reference value after each load power transient but instead is changed inversely proportional to the load. Different solutions have been proposed for open-loop or closed loop droop control of frequency. Open-loop operation is possible since the inverter-filter-load system does not influence or alter the generator frequency. Since the output is common during paralleling, closed loop voltage control cannot be implemented. On the other hand, using open loop generation of the voltage introduces problems related to the system non-linearity and voltage drop. Implementation of the droop coefficients method requires calculation of the power components at the point of load. This is usually based on direct measurement of the voltage and current at the inverter output. Operation with isochronous frequency helps the measurement system within the controller. Measurement of output voltage RMS voltage, frequency and instantaneous phase are more accurate when done at a fixed and known frequency than while subjected to large variation ranges. If any closed-loop voltage and/or frequency control system (based on communication between inverters) is used, accurate measurement becomes very important. Finally, it is easier to achieve hot-swap (connection of a power inverter while another one is already working on the bus) when the operation is based on isochronous frequency.